1. Field of Invention
This invention relates to a process for formation of microcapsules. More particularly, this invention relates to an improved process for microencapsulation by interfacial reaction. The invention is particularly applicable to encapsulations wherein the continuous phase is the aqueous phase, and the aqueous phase reactant is a polyamine. The oil phase reactant is a polyisocyanate.
2. Description of the Prior Art
United Kingdom patent 950,443 MacKinney and U.S. Pat. No. 3,424,827 Ruus are some of the early teachings relating to interfacial encapsulation.
Early capsules for carbonless business forms were made using polyamines and acid chloride reactants. These processes, however, had acid generating side reactions undesirable in the presence of acid sensitive dyes.
Later, capsules were made from aliphatic polyamines and aliphatic polyisocyanates which react at the oil water interface to produce a polyurea wall. This process eliminates the acid generating side reactions. The use of the all aliphatic reactants appears to eliminate the slow discoloration which occurs with aromatic reactants. U.S. Pat. No. 4,761,255 Dahm describes a semi-continuous process to produce microcapsules using such reactants.
Two Monsanto patents, U.S. Pat. Nos. 4,280,833 and 4,563,212, describe increased solids for interfacial encapsulation processes by use of polyanionic emulsifiers. These processes, while perhaps useful for pesticide application where larger capsules and slow release are paramount, are not particularly suitable for microencapsulation for carbonless applications. In these processes, the unavoidable hydrolysis and decarboxylation of isocyanate reactant to amino, plus the presence of amino dyes renders the oil droplets slightly cationic. Anionic polymers bind to this cationic surface, forming a layer impeding emulsification and, after emulsification, diffusion of the polyamine reactant to the interface.
U.S. Pat. No. 4,428,978 teaches production of microcapsules by interfacial polyaddition of polyisocyanate and a hydrogen active compound. The polyisocyanate is an isocyanurate-containing aliphatic polyisocyanate. High encapsulation solids are taught, obtained by lowering the suspension pH to or below 7 after polyamine addition.
Improved processes for producing high solids aqueous suspensions of microcapsules would be of commercial significance.